But the benchmark's reliance on mega-cap tech has come at a
price. Market pessimists have frequently cited the
highly-concentrated gains as a negative driver, arguing that
while the ride higher is enticing, any unwinding can be
swift and brutal.

After all, just 10 companies have accounted for almost half of
the benchmark's return this year.

It's all a misconception, argues JPMorgan, who doesn't think the
concentrated market move is anything out of the ordinary.

The contribution of tech titans Facebook, Apple, Amazon,
Microsoft and Google is in just the 55th percentile when measured
against other groups of top-five contributors going back almost
30 years. When the list is expanded to 25 companies, the dynamic
seems even more normal, with the contribution dropping into the
51st percentile, JPMorgan data show.

"While equity leadership has been narrowing into growth stocks,
history suggests it is not at extreme levels and not far from
historic norms," a group of JPMorgan equity strategists led by
Dubravko Lakos-Bujas wrote in a client note.

The
contribution of the top 5 stocks YTD is in line with history, and
below average on a trailing 60-day basis.JPMorgan

While stock bears have been stressing caution, some have gone as
far as to
compare the current environment to the dotcom bubble. Once
again, JPMorgan doesn't agree. The firm argues that
market-leading stocks are contributing far more to earnings than
they did during the tech bubble, making it a far less precarious
situation.

While the top five stocks of 2017 have accounted for a
similar percentage of market cap, they've also contributed 17% to
forward sales growth, and 12% to earnings expansion. That's a big
improvement over the dotcom era, where the five leaders spurred
growth of just 8% for both metrics.

Piggybacking off their lack of concern, JPMorgan is intrigued by
growth stocks, which they see grinding higher. With profit
expansion forecasts already quite lofty, and with economic
indicators peaking, there are fewer places to find growth
opportunities. That means investors will be increasingly willing
to pay a premium for growth stocks, the firm says.

So where does JPMorgan see the market going from here, with all
of that considered? They think it'll grind sideways, with
capital churning back and forth between styles and sectors.

The so-called rotations are already happening all the time. On
June 9, tech stocks in the S&P 500 dropped 2.7%. But that
money mostly stayed in the stock market, flowing into energy and
financial shares. The benchmark index ended up falling less than
0.1% for the session.

JPMorgan highlights a measure called dispersion — which assesses
how widely distributed market returns are — as particularly
indicative of this phenomenon. By their tally, it's the highest
since 2006 on a sector basis.